Single phase induction motors require an arrangement to generate a rotating magnetic field and produce a rotational torque to accelerate the motor towards its running speed. A common arrangement uses a start or starting winding in the motor spaced at an angle to the main winding and in which a starting current, displaced in phase from the main winding current, is arranged to flow during acceleration to operating speed.
The start winding is usually arranged to operate with a current that leads the current in the main winding and that current lead may be achieved by designing the winding to be inherently more resistive than the main (inductive) winding [ie. resistive start] or by connecting a capacitor in series with that winding to control the phase shift and achieve even stronger starting torques [ie. capacitive start]. In motors having a start winding designed to be disconnected once the motor has reached a designed speed this intermittent operation allows this winding to have an increased short-term power rating and reduced manufacturing cost.
As the person skilled in the art would know there are arrangements for achieving the disconnection once a certain speed has been reached, usually at around 80% of synchronous speed, and for reconnecting it again if the motor slows.
Purely mechanical (centrifugal) switches are still in use today (U.S. Pat. No. 4,658,196). Electromechanical switches (relays) (U.S. Pat. No. 3,624,470) with their coils directly energized by the motor winding currents or voltages have also been used. The main problem with all mechanical solutions is reliability. Since the switching currents are quite large, mechanical contacts will always have a limited life.
Because relay coils designed for operation on AC are relatively expensive to produce, and have large spreads in their characteristics, more precise control can be obtained using relays with DC coils.
Other examples like rectified AC signals as well as the use of various rotational speed transducers, typically responsive to magnetic or optical signals are known by the person skilled in the art. The transducer signals, truly proportional to the motor speed are processed to yield a simple on/off drive to the DC relay.
Again there have been many arrangements described for electronically processing the basic motor winding currents or voltages to achieve greater precision than possible using relays directly energized by these signals.
The availability of solid-state switches in the 60s enabled the replacement of the mechanical switch contacts, which were subject to arcing and mechanical wear-out, by triacs or related semiconductor devices.
Continual refinement and cost reduction in motor design has seen a corresponding requirement for precision in the speed of operation in controlling the start winding. Existing arrangements using electronic sensing and a solid-state switch have response time limitations and the objective of this invention is to achieve an arrangement that is still simple and inexpensive yet achieves the very fast and precise control of the motor speed at which the switching of the start winding will occur.
It becomes important to recognize that the motor parameters and supply reference signals being monitored are all AC signals, predominantly sinusoidal in shape, having as their fundamental the AC mains frequency.
Whenever such AC voltage or current signals are being referenced in prior art it is important to understand that it is necessary to specify whether the reference is, for example, to their peak, average, or rms amplitudes and the way in which that value might be determined and used by the system. It is also important to recognize that, at any instant, an AC signal has only one instantaneous value.
In most prior art cases the AC signal is rectified and a DC voltage, proportional to the AC signal, stored on an integrating capacitor. The process is generally described as ‘peak detection’ or ‘peak rectification’ but for practical reasons the derived DC value will generally lie somewhere between the rectified average and true peak value of the sine wave signal.
This process requires the detection of multiple sinusoidal peaks, followed by a filter having sufficient time-constant to maintain the “peak” voltage between signal peaks. In a 60 Hz system, each peak occurs at 16.7 ms intervals (20 ms for a 50 Hz system). The charging or the discharging of the integrating capacitor will generally be characterized by the need to select a fixed charge or discharge time constant. If a filter time-constant of say 5 times the signal repetition rate is required, then the response time of the output signal from the peak rectifier and filter is of the order of some 80 to 100 ms.
Further, noise or other effects causing distortions of the nominal sine wave shape also requires processing of several cycles of the AC signal before any reasonably representative DC value can be derived.
Given that, for modern motors, the start circuit must power-up and respond within less that 400 ms (the typical time for the motor to reach approx. 80% of full speed), the delay produced by such an arrangement becomes unacceptable. Indeed this prior art principle teaches away from the current invention, which instead preserves the AC signals, and processes them in real-time, using both amplitude and phase information of the measured parameters.
U.S. Pat. No. 5,296,795 (1994) provides a representative example of a prior art control system using rectification of the sensed signals. In this arrangement the start winding voltage has, as its reference, the mains supply so the start winding is sensed using two resistive dividers, one for the voltage at each end of the start winding (R3/4 and R5/6), and the AC difference is then rectified (AMP 1) and the resulting DC value is stored on an integrating capacitor (C3) for use in the switching decisions. There is also an arrangement for detecting the amplitude of the mains (BUF 1) that is based on resistive attenuation (R3/4) of that AC voltage followed by conventional ‘peak’ rectification with the resultant DC stored on an integrating capacitor (C4). That DC value becomes one parameter that will also be used in determining the switching decisions.
Therefore there still remains a need in the field of induction motors to provide an improved start switch device arrangement of the motor start circuit for it to better serve the purpose of controlling the applied voltage or current flow through the start winding.
Accordingly it is an object of this invention to provide a motor start circuit for an induction motor with a main winding and a start winding, which are supplied with alternating current or voltage from a mains power supply, with such an improved start switch device control arrangement.
Further objects and advantages of the invention will become apparent on the complete reading of this specification.